Weapons like the AR15/M16/M4 are capable of firing in full automatic mode but seldom are for various reasons. Chief among those reasons is the rapid accumulation of heat first in the barrel and then throughout the rest of the firing mechanisms. Among the problems caused by accumulated heat not being rejected from the rifle are: (1) thermal expansion causing lock up of the metal pieces in the mechanisms that extract spent shells from the chamber and load new shells from the magazine, (2) auto-discharge of the cartridge when loaded into the hot chamber without the firing pin striking the cartridge primer (aka ‘cook-off’), (3) rupture of the weakened barrel and (4) rupture of the weakened gas tube, which transfers hot combustion gases from the barrel first through the gas block and then into the upper receiver to cycle the action.
Designing a rifle to ameliorate elevating barrel temperatures has in the past conflicted with the need for a light weight rifle that can be readily carried by a single person. A major portion of the total rifle weight, which ranges from 6 to 8 pounds for AR15/M16/M4, is contributed by the barrel itself. Elevated barrel temperatures also cause severe degradation in rifle accuracy. As the metal barrel gets hot, it becomes less rigid, flexing more when fired and causing the bullet trajectory to be erratic. Further complicating the design of a barrel cooling system for weapons like the AR15/M16/M4 is the size, function and location of the gas block, which is located typically near the midpoint of the barrel overall length.